Little is known about the human upper thermoneutral zone and whether humans have an upper critical temperature. Some authors suggest that there may be (Withers, 1992; Faerevik et al., 2001; Pallubinsky et al., 2019).  There are few studies having investigated resting metabolic rate (RMR) at higher temperatures and even fewer on the effects of associated physiological variables. To investigate potential changes in respiratory gas exchange, cardiovascular variables and body temperature during rest at high temperatures, thirteen healthy individuals (µ= 32.7 ± 8.2 years; 7 females) rested in five environmental conditions, each one hour in length, of increasing environmental severity (i.e. increasing temperature and relative humidity (RH)). Participants experienced a baseline condition of 28℃ and 50% RH, and 40℃ and 50℃ both at dry (25% RH) and wet (50% RH) conditions which were ranked from 1 (baseline) to 5 (50℃ 50% RH). All participants repeated conditions 2 to 5. At 20, 40 and 60 minutes into the experiments expired air was collected and analysed using the Douglas Bag method. Core and skin temperatures, heart rate, blood pressure, movement levels and breathing rate were recorded throughout each condition. Some Douglas bag data were discarded due to the rapid saturation of drierite required to remove water from the sample passed to the gas analyser. A repeated measures one-way ANOVA indicated evidence for differences in metabolic rate between conditions (F(4,37) = 2.38, p = 0.070; n = 8); a Tukey’s HSD post-hoc test revealed a statistically significant increase in VO2 between baseline and condition 5 (0.091 lO2 min-1 ±0.030(SEM), p=0.033). Mean metabolic rate per condition ranged from 0.20 lO2 min-1 ±0.020(SEM) to 0.29lO2 min-1 ±0.026.  Other physiological variables also indicated a response to this heat stress. Skin temperature and heart rate increased whilst diastolic blood pressure fell with rising core temperature. A one-way ANOVA followed by a Tukey’s HSD post-hoc test confirmed a marked step change in these variables occurring at condition 5 (n=13, F(8,106) = 10.73, p = 0.000; µ=3.81℃, SEM=0.39, p=0.000; µ=36.95bpm, SEM=5.44, p=0.000; µ=11.02mmHg, SEM=5.28, p=0.038; µ=1.15℃, SEM=±0.14, p=0.000, respectively). A small number of previous studies offer tentative evidence for an increase in metabolic rate in naked humans above about 40℃ (Faerevik et al., 2001). However, many individuals do not show any increase (Pallubinsky et al., 2019) and none of these studies controlled for movement levels, which might well represent a confound. This study found evidence for increased RMR at 50℃ 50%RH compared to baseline, a condition not tested in the aforementioned research. Further analyses of the data will uncover whether movement levels explain this increase in RMR and indicate whether breathing rate varied across conditions or were affected by the breathing masks worn during expired air collection.